1. Field of the Invention
The present invention relates to a heart stimulator of the type having a pulse generator for generating and delivering stimulation pulses to a heart, at least one detector for sensing events in the heart and a control device for controlling the pulse generator's emission of stimulation pulses according to events sensed by the detector, the pulse generator emitting stimulation pulses if a spontaneous heart rate, among events sensed by the detector, drops below a defined basic rate.
2. Description of the Prior Art
A heart stimulator of the type described above is generally referred to as "inhibiting," i.e., it inhibits emission of stimulation pulses as long as the heart naturally operates at an adequate beat rate. The heart stimulator can be for single chamber pacing or dual chamber pacing. The operating mode is designated with a three-letter alphabetic code in which the first letter designates the site of stimulation, A therefore designating an atrial site, V designating a ventricular site and D designating both atrial and ventricular sites, the second letter designates the site of sensing and the third letter designates the response to signals, I designating inhibition. Thus, AAI is a heart stimulation mode in which the atrium is stimulated and sensed and in which stimulation pulses are inhibited when spontaneous events are detected. VVI is the same mode although with a ventricular site, and DDI applies to both the atrium and the ventricle. DDI requires a dual chamber stimulator, whereas AAI and VVI can be programmed in both single chamber and dual chamber stimulators.
Stimulation pulses are emitted at a basic rate when the spontaneous heart rate drops below the basic rate. The basic rate is normally 60 to 80 beats/min. The heart stimulator can be programmed with hysteresis to prevent competitive stimulation when the spontaneous heart rate oscillates around the defined basic rate. The hysteresis rate is a rate slower than the basic rate. The spontaneous heart rate must then drop below this slower rate before stimulation pulses are emitted.
One type of pathological condition in the heart, the vasovagal syndrome, is manifested in a sudden drop in the spontaneous heart rate, causing the patient to faint, followed by restoration of the normal rate in a few seconds or minutes. The speed with which the spontaneous heart rate drops varies from one patient to another. Patients with this condition are generally provided with a heart stimulator which stimulates the heart when the spontaneous rate plummets to keep the patient from fainting. In known heart stimulators currently available, the spontaneous heart rate, once the heart stimulator has begun stimulation, must exceed the stimulation rate before the inhibiting function is reactivated so as to "silence" the stimulator. Thus, stimulation of the heart could still continue, even after the spontaneous heart rate has normalized.
Since people with this type of heart condition do not normally have any other heart defects, limiting stimulation to the briefest periods possible would be advantageous.